In recent years, enthusiastic research has been directed to rare gas discharge lamp devices using a dielectric barrier discharge for use in backlights for liquid crystal displays or the like. Rare gas discharge lamp devices are desired for the reason that they do not exhibit a decline in luminous efficiency by a rise in mercury temperature and for environmental reasons, since they do not use mercury.
An example of rare gas discharge lamp devices using a dielectric barrier discharge is disclosed in JP 5(1993)-29085A. In the rare gas discharge lamp device disclosed in this publication, an internal electrode is provided in one end portion of an arc tube containing a rare gas, and an external electrode is provided axially on an outer surface of the arc tube. By applying a voltage to both the electrodes, phosphors in the tube are excited and emit visible radiation.
However, when this discharge lamp device is lit by a small tube current, light is not emitted throughout the whole arc tube, resulting in a partial discharge on the internal electrode side. In order to allow light to be emitted throughout the arc tube, a higher tube current is required, which leads to an increase in power consumption by the lamp, a rise in tube wall temperature, and a shortened lighting life due to an increase in sputtering of the internal electrode. Moreover, brightness declines as the distance between the internal and external electrodes becomes greater. More specifically, although high brightness can be achieved near the internal electrode where phosphors are excited easily, it becomes more and more difficult to cause excitation and thus the brightness declines gradually as the distance from the internal electrode becomes greater. As a result, brightness varies depending on the position in the arc tube.
As a solution to these problems, JP 2001-210276 A discloses a discharge lamp device in which a helical external electrode is provided on an outer surface of the arc tube containing a rare gas. The helical electrode provides a condition such that the external electrodes are arranged intermittently in the direction of the tube axis, which allows uniform charge to be obtained throughout the arc tube. Thus, the problems as mentioned above are solved.
However, in order to provide a helical external electrode, it is not easy to attach such an electrode to the arc tube. Practically, a configuration is required for positioning and holding the helical external electrode accurately relative to the arc tube and the internal electrode, which contributes to an increase in manufacturing costs. For these reasons, the external electrode that has the same function as the helical external electrode and is attached to the arc tube easily and reliably is desired.
On the other hand, to make backlights thinner, arc tubes with a smaller diameter (outer diameter), that is, thinner arc tubes, are desired. When the inner diameter of the arc tube is reduced with the tube diameter, an emission area decreases and the lamp efficiency declines. Such a tendency is shown in FIG. 7. Thus, even when the tube diameter is reduced, the inner diameter should be maintained, which means the wall thickness of the arc tube has to be reduced relatively. However, when the wall thickness of the arc tube is reduced, a lamp current increases, and thus the discharge becomes unstable and the lamp efficiency declines. FIG. 8 shows such a tendency for the lamp efficiency to decline with a reduction in wall thickness of the arc tube. This tendency is attributed to the fact that the capacitance of a dielectric layer formed by the glass wall of the arc tube increases as the dielectric layer becomes thinner. Thus, in order to reduce the diameter of the arc tube, it is desirable to provide a configuration for suppressing an increase in the capacitance of the dielectric layer.